This invention relates to a power assisted vehicle steering system. It has hitherto been proposed and is now recognised as a desirable feature of power assisted vehicle steering that the power assistance which is provided should decrease as vehicle speed increases and vice versa. By this proposal the steering system may provide very little or no power assistance when the vehicle is at high speed and full power assistance will be provided when the vehicle is at low speed, such as during a parking manoeuvre. Conventionally, a power assisted steering system has a steering gear, a fluid pressure operated servo motor device for the gear, a control valve controlling fluid pressure to the servo motor device in response to a steering manoeuvre whereby the servo motor responds to assist that manoeuvre, and a fluid pressure supply to the control valve, this supply usually being hydraulic fluid pressurised by a pump. A widely accepted manner of reducing power assistance as vehicle speed increases is to provide the control valve for the servo motor device with a means that is responsive to vehicle speed and reacts to increase the resistance of the control valve to operate in response to a steering manoeuvre as the vehicle speed increases--an example of this proposal is to be found in our G.B. Specification No. 1,456.901. By further proposals, as disclosed for example in Japanese Specification No. 58-71262 and G.B. Specification No. 2,014,795, the fluid pressure supply to the control valve is determined by a regulating or throttle valve which is responsive to vehicle speed whereby, for example, as disclosed in G.B. Specification No. 1,393,046 the fluid pressure within the servo motor is "drained off" to reduce the power assistance as vehicle speed increases. These prior proposed systems have suffered from problems of complexity, reliability, size and cost. More particularly, where the aforementioned regulating valve throttles the supply of fluid under pressure to the control valve under control of an electrical solenoid, the accuracy and consistency in adjustment of the regulating valve by a speed responsive solenoid leave much to be desired, particularly becuase the characteristics of a solenoid valve are frequently other than consistent. Also the operation of the solenoid valve may be impaired by pressure variations to which it is subjected from the fluid supply during its adjustment affected the characteristics required of the speed responsive solenoid control (for example where the speed responsive solenoid regulating valve is arranged as disclosed in G.B. Specification No. 2,014,795). It is an object of the present invention to alleviate the above mentioned disadvantages in the provision of a power assisted vehicle steering system of the kind generally discussed above.